Radiographic evidence of metaphyseal sclerosis secondary to canine distemper virus: 4 cases in juvenile dogs

Abstract Background Metaphyseal sclerosis secondary to canine distemper virus has been described histopathologically, but its radiographic appearance has not been described. Objectives Describe the radiographic appearance of metaphyseal sclerosis secondary to canine distemper virus in juvenile dogs as distinct from metaphyseal osteopathy (formerly called hypertrophic osteodystrophy). Animals Four dogs (2 intact females and 2 intact males) between 2.5 and 4 months of age presented to 2 different veterinary teaching hospitals. Methods Retrospective case series in which definitive diagnosis of canine distemper virus based on antemortem positive reverse transcription‐polymerase chain reaction (RT‐PCR) result or necropsy was required. Results All 4 dogs were presented for evaluation of neurologic abnormalities, respiratory signs, and lethargy; 2 dogs had gastrointestinal signs and ocular abnormalities. Radiographs on all patients featured multifocal, symmetric, metaphyseal sclerosis, with no evidence of lysis or changes to the adjacent growth plate. The metaphyseal sclerosis was most apparent at the proximal humeral diaphyses and other included long bones. Diagnosis of distemper was confirmed by necropsy (2 of 4 dogs) or positive RT‐PCR results (2 of 4 dogs). Three dogs were euthanized because of progressive illness, and 1 dog was lost to follow‐up. Conclusion and Clinical Importance Identification of metaphyseal sclerosis on radiographs during diagnostic evaluation of young dogs should lead to a clinical suspicion of canine distemper virus infection. Sclerosis identified secondary to canine distemper virus is distinct from the necrosis and inflammation of metaphyseal osteopathy.


| INTRODUCTION
Canine morbillivirus (canine distemper virus [CDV]) produces a wide range of clinical findings, including fever, lethargy, inappetence, vomiting, diarrhea, respiratory signs, conjunctivitis, neurologic signs, and hyperkeratosis of the footpads and nasal planum. 1,2 A diagnosis of CDV is not always straightforward and usually is based on clinical signs, physical and neurologic examination findings, or both, and additional diagnostic test results. The most commonly utilized tests to definitively diagnose CDV are RT-PCR or histopathology. 1,2 The RT-PCR tests can be performed on a variety of body fluid or tissue samples, including blood, urine, respiratory secretions, conjunctival swabs, or tissue biopsy samples. Histopathology of cutaneous lesions (eg, keratinized foot pads) or frequently affected tissues (eg, lungs, bladder, and lymph tissue) can identify characteristic intracytoplasmic or intranuclear viral inclusions. 1,2 The turnaround time for these test results, when samples are collected antemortem, can delay appropriate treatment. Identifying additional diagnostic or clinical criteria that support a diagnosis of distemper could facilitate diagnosis and appropriate treatment earlier in the course of disease, especially in cases of young animals with suspicion of an infectious disease.
In a previous study, 3 CDV antigen was demonstrated in long bone metaphyses (ie, humerus, radius, ulna, femur, tibia, and fibula) between 5 and 36 days after infection. Additionally, homogenous band-like increased density of bone (sclerosis) was identified grossly just beneath the growth plate. Lesions in that study were histopathologically characterized by necrosis of osteoclasts, osteoblasts, and marrow cells as well as by persistence of primary spongiosa. 3 These features are different from the previously described disease process of metaphyseal osteopathy, formerly referred to as hypertrophic osteodystrophy (HOD). Metaphyseal osteopathy histopathologically is characterized by a band of neutrophilic inflammation and necrosis within the primary spongiosa that can progress to lysis and infarction and, in chronic cases, periosteal and extraperiosteal woven bone proliferation at the metaphyses and physes. 4,5 To our knowledge, the identification of sclerosis secondary to CDV using radiography has not been described in the literature. Our objective was to describe the radiographic appearance of metaphyseal sclerosis secondary to canine distemper virus in 4 juvenile dogs and its distinction from metaphyseal osteopathy. Thoracic radiographs acquired on presentation disclosed a diffuse mild unstructured interstitial pulmonary pattern, considered excessive for the patient's young age ( Figure 1A). Additionally, symmetrical welldefined sclerosis of the included proximal humeral metaphyses was noted. The proximal humeral physes themselves were considered normal with no evidence of lysis or periosteal proliferation (Figure 2A).

| MATERIALS AND METHODS
The dog was treated with crystalloid fluids, antibiotics, and antiseizure medications as deemed appropriate by the supervising clinician. Because of persistent muscle tremors, persistent dull mentation, recumbency, and lack of improvement the next day, the owners elected euthanasia. A distemper RT-PCR assay on submitted urine was negative. A necropsy was performed.
Necropsy identified opaque, pink to white, linear bands of hard, dense, sclerotic bone, 1 to 2 mm wide in the proximal and distal metaphyses of the long bones of both pelvic and thoracic limbs ( Figure 3).

| Case 2
Nine days after presentation of case 1, another puppy from the same household (a 4-month-old female intact French Bulldog) was presented to the University of Tennessee Small Animal Hospital for evaluation of progressive shaking and increased respiratory effort. This puppy had been acquired recently from a different breeder and also was reported to be "partially vaccinated." The dog had been brought F I G U R E 1 Lateral radiographs (all in left lateral recumbency) of Cases 1 through 4 showing diffuse mild unstructured interstitial pattern throughout the pulmonary parenchyma. (A) Case 1 has a generalized unstructured interstitial pattern throughout the lungs, more conspicuous in the caudodorsal lung fields and superimposed with the cardiac silhouette. There is a microchip within the lateral thoracic soft tissues, superimposed with the cranial thorax. (B) Case 2 generalized unstructured interstitial pattern in a brachycephalic patient. The patient's thoracic cavity is shortened and cardiac silhouette appears widened because of breed-related conformation. There is evidence of a sliding hiatal hernia in caudodorsal thorax (asterisk), as well as mild gas dilation of the mid thoracic esophagus (carets). There are also caudal thoracic vertebral malformations, considered an incidental breed-related finding. (C) Case 3 generalized unstructured interstitial pattern throughout the lungs, more conspicuous in the caudodorsal lung fields. (D) Case 4 diffuse mild bronchial and unstructured interstitial pulmonary pattern, and ventrally dependent, focal, marked unstructured interstitial pattern (arrowheads) superimposed with the heart apex to the referring veterinarian a few days before for evaluation of lethargy. At that time, thoracic radiographs showed pulmonary infiltrates, and amoxicillin-clavulanate potassium (Clavamox, Zoetis, Inc) was prescribed. The dog had initially improved on the antibiotics, but the owners noted progressive shaking and tremors over the 2 to 3 days before presentation.
At presentation, the dog was mildly dyspneic with increased respiratory rate and effort. Lung sounds were harsh bilaterally. On physical examination, the dog had mild muscle tremors but no other neurologic or muscular abnormalities. Other physical examination abnormalities included mildly increased heart rate of 126 beats/min (reference interval, 60-120 beats/min) and increased respiratory rate of 84 breaths/min (reference interval, <40 breaths/min). The dog was admitted to the hospital and started on enrofloxacin injectable (Baytril, Elanco US, Inc, Greenfield, Indiana) and amoxicillin sodium sulbactam sodium injectable (Unasyn, New York, New York) and placed in oxygen at 40% FiO 2 (fraction of inspired oxygen).
Thoracic radiographs the next day showed a generalized moderate unstructured interstitial pulmonary pattern ( Figure 1B Figure 6). Urine CDV RT-PCR testing was negative and blood lead concentration was normal.
The dog's respiratory signs and laboratory abnormalities improved somewhat by the third day of hospitalization, and the muscle tremors were static. The owners elected to take the dog home on the fourth day of hospitalization. They returned the dog to the hospital the same evening because of deteriorating condition, particularly worsening respiratory distress. Repeat thoracic radiographs showed a progressively severe unstructured interstitial pattern diffusely affecting the lungs, with focal alveolar patterns in the cranioventral lung fields, raising concern for concurrent aspiration pneumonia.

| Case 3
A 2½-month-old male intact Labrador mix was presented to the Uni- the distal radius and ulna. 6,7 It has been anecdotally reported to be associated with CDV vaccines. 4,8 The cases in our report do not display the radiographically aggressive changes described in metaphyseal osteopathy, characterized by bands of lysis affecting the metaphyses and parallel to the physes (frequently referred to as the "double physis sign"). 9,10 Additionally, there was no osseous proliferation extending around the metaphysis or physis, which also has been described in metaphyseal osteopathy, usually when its more chronic. 9 Two prior studies have described metaphyseal lesions histologically in dogs with CDV infection. In 1 study, 3  The long bone metaphyseal changes described in our report have a similar appearance to the previously described metaphyseal lesions formed secondary to lead toxicity (seen in both humans and animals), but blood lead concentration was measured in 1 of the dogs and was normal. The radiographic osseous lesions secondary to lead poisoning are described as radiopaque lines or bands within the metaphyses, adjacent to the growth plates. [12][13][14] Lead precipitates as insoluble salts in bone and represents a type of long-term storage in the bone. 12 The clinical signs of lead poisoning include neurologic and gastrointestinal signs, which overlap with clinical signs of CDV, and therefore a thorough history and physical examination should be performed to evaluate the possibility of lead poisoning when metaphyseal sclerosis is identified radiographically.
Canine distemper is an important differential diagnosis for non- Necropsy of dog 2 identified a small thymus and multifocal lymphoid depletion microscopically throughout organs. This finding is consistent with lymphocytic viral-induced apoptosis, as previously reported in lymphocytes within blood, bone marrow, thymus, spleen, and other organs. 1,17,18 This mechanism also explains the leukopenia noted in cases 2, 3, and 4.
Treatment for CDV is typically supportive and directed at managing clinical signs. In our study, 3 of the 4 dogs were euthanized because of the severity of clinical signs, and the 4th dog was lost to follow-up. It is likely that mildly symptomatic dogs with distemper may go undiagnosed if veterinary care is not sought or the response to initial supportive treatment is sufficient.
Neurologic abnormalities may occur in up to 30% of infected dogs, and can occur weeks after the onset of clinical signs or recovery from systemic illness. 1,17 Neurologic signs also can develop later in life. 1,17 All dogs in our report had neurologic signs, ranging from mild muscle tremors to debilitating tonic-clonic seizures. Dogs 3 and 4 had decreased tear production, consistent with keratoconjunctivitis sicca, previously described as a possible sequela to distemper and hypothesized to be caused by damage to the lacrimal glands. 1 We describe a previously unpublished radiographic finding of metaphyseal sclerosis associated with CDV infection. Our report indicates that metaphyseal sclerosis may support the diagnosis of CDV infection in young dogs suspected of infectious disease. Furthermore, the sclerosis identified secondary to CDV is distinct from the necrosis and lysis of metaphyseal osteopathy.